Aromatic dicarboxylic acids are well known starting materials for making polyester resins, which polyester resins are used widely as principal polymers for polyester fibers, polyester films, and resins for bottles and like containers. For a polyester resin to have properties required in many of these uses, the polyester resin must be made from a polymer grade or "purified" aromatic acid. Polymer grade or purified terephthalic acid and isophthalic acid are the staring materials for polyethylene terephthalates and isophthalates, respectively, which are the principal polymers employed in the manufacture of polyester fibers, polyester films, and resins for bottles and like containers. Similarly, polymer grade or "purified" naphthalene dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid, are the starting materials for polyethylene naphthalates, which can also be employed in the manufacture of fibers, films and resins.
A purified terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid can be derived from a relatively less pure, technical grade or "crude" terephthalic acid, isophthalic acid or "crude" naphthalene dicarboxylic acid, respectively, by purification of the crude acid utilizing hydrogen and a noble metal catalyst, as described for terephthalic acid in commonly assigned U.S. Pat. No. 3,584,039 to Meyer. In the purification process, the impure terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid is dissolved in water or other suitable solvent or solvent mixture at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, which conventionally is palladium on a carbon support, as described in commonly assigned U.S. Pat. No. 3,726,915 to Pohlmann. This hydrogenation step converts the various color bodies present in the relatively impure phthalic acid or naphthalene dicarboxylic acid to colorless products. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in commonly assigned U.S. Pat. No. 4,405,809 to Stech et al.
Puskas et al., in commonly assigned U.S. Pat. Nos. 4,394,299 and 4,467,110 disclose the use of a combination noble metal catalyst, for example, a palladium/rhodium catalyst on a porous carbonaceous surface, for hydrogenation of aqueous terephthalic acid solutions. These two patents also show the use of a rhodium-on-carbon catalyst under reducing conditions and review various heretofore known methods of preparing a Group VIII metal catalyst having activity and selectivity suitable for the purification of terephthalic acid by hydrogenating its principal impurity, 4-carboxybenzaldehyde, to para-toluic acid.
However, para-toluic acid is also an impurity that must be removed from the hydrogenated aqueous terephthalic solution. While such removal can be achieved to a large extent owing to the greater solubility of para-toluic acid as compared to terephthalic acid in water, nevertheless, substantial mounts of para-toluic acid are trapped within purified terephthalic acid crystals as the hydrogenated terephthalic acid solution is crystallized to recover purified terephthalic acid.
To avoid disadvantages attendant to the separation of para-toluic acid from the terephthalic acid crystals, it has been proposed to decarbonylate 4-carboxybenzaldehyde in aqueous solutions to benzoic acid in the presence of a palladium-on-carbon catalyst but without the simultaneous hydrogenation of the other impurities that may be present in aqueous solutions of crude terephthalic acid since benzoic acid is more soluble in water then para-toluic acid. See, for example, commonly assigned U.S. Pat. No. 3,456,001 to Olsen. This proposed decarbonylation of 4-carboxybenzaldehyde to benzoic acid produces, however, equimolar mounts of carbon monoxide, a well-known poison for the noble metals such as palladium.
Kimura et al., U.S. Pat. No. 4,201,872, describes using a catalyst of palladium supported on active carbon to effect decarbonylation of the 4-carboxybenzaldehyde impurity in crude terephthalic acid obtained by liquid phase oxidation of para-xylene in water at high temperature under high pressure. Carbon monoxide which is formed by decarbonylation of the 4-carboxybenzaldehyde and dissolved in the liquid phase, was found to poison the catalyst of palladium supported on active carbon which resulted in a decrease in catalyst activity for the decarbonylation of 4-carboxybenzaldehyde. In an attempt to minimize catalyst poisoning, Kimura et al. propose to carry out the decarbonylation at relatively low process pressures so as to minimize dissolved carbon monoxide concentration in the liquid reaction medium. According to Kimura et al., carbon monoxide dissolved in the liquid phase can be removed from the liquid phase to a gaseous phase by decreasing the pressure over the catalyst bed in the decarbonylation step thereby prolonging the lifetime of the catalyst. The generated carbon monoxide is purged from the reactor as a gas.
Kimura et al. describe a process consisting essentially of contacting an aqueous solution of crude terephthalic acid obtained from the oxidation step with a catalyst of palladium supported on active carbon to effect decarbonylation under a limited pressure of an ambient atmosphere comprising steam and carbon monoxide; and cooling the solution to effect separation by crystallization of pure terephthalic acid from solution thereby leaving water soluble impurities comprising benzoic acid formed by decarbonylation in solution. The mount of carbon monoxide present is only that mount derived by the decarbonylation of the 4-carboxy-benzaldehyde. Process pressures also must be controlled within a closely defined pressure range. The limited pressure is in a range from the vapor pressure (kg/cm.sup.2) of the aqueous solution at the reaction temperature of 200.degree. C. to 320.degree. C., up to pressure equal to the sum of said vapor pressure with no more additional pressure than 5 kg/cm.sup.2 or, optionally, with additional pressure of only 3 kg/cm.sup.2. U.S. Pat. No. 4,201,872 contains no mention of titanium dioxide-supported purification catalysts or of hydrogenation prior to recovery of terephthalic acid by crystallization.
Japanese Patent Application No. 145922/76 describes using a catalyst of Group VIII metals and/or their oxides for purification of aromatic dicarboxylic acids in a process step characterized by passing aqueous solutions of crude aromatic dicarboxylic acid through a bed of catalyst which was not allowed to come into contact with molecular hydrogen, and another process step of allowing the solution to contact catalyst and molecular hydrogen. In an attempt to minimize catalyst poisoning, applicants propose that deactivated catalyst of palladium impregnated activated carbon, i.e., catalyst which lost hydrogen-addition activity by having been used in the purification of terephthalic acid by hydrogen addition, should be suitable for the first process step. Japanese Patent Application No. 145922/76 contains no mention of any titanium dioxide-supported purification catalyst.
Carbon is conventionally used as the support material for the noble metal in the catalyst employed in the aforesaid purification processes. A common disadvantage of the use of a carbon support is that carbon fines are often generated during commercial operations. The generation of such fines can be minimized but generally cannot be completely avoided. During the subsequent esterification process, such particulates introduced with the particular purified acid, for example, terephthalic acid, isophthalic acid or 2,6-naphthalene dicarboxylic acid, can plug filters and thereby cause interruptions in the process. Other such particulates that bypass the filter may be incorporated into the resulting polyester fiber or film and cause fiber breakage or film distortion.
For this reason, it is highly desirable to use other materials as the support material in the catalyst employed in the aforesaid purification method. However, because of the highly corrosive conditions under which the aforesaid purification is performed, it has proven difficult to develop suitable non-carbon catalyst supports for use in the purification catalyst. For example, as indicated in Meyer, U.S. Pat. No. 3,594,039 in column 5, lines 10 to 14, hot aqueous solutions of terephthalic acid dissolve supporting materials such as natural and synthetic alumina, silica, silica-alumina, kieselguhr, calcined clays, zirconium supports and other metal oxides and metal salt containing supports.
M. Bankmann, R. Brand, B. H. Engler and J. Ohmer, "Forming of High Surface Area TiO.sub.2 to Catalyst Supports," Catalysis Today, Vol. 14, pages 225-242 (1992), contains an extensive discussion of the use of titanium dioxide having a high surface area as a catalyst support. The article (which was previously presented in a substantially identical form by R. Brand at the Fall, 1991, American Chemical Society meeting) indicates that the titanium dioxide must have a high surface area in order to be a suitable catalyst support and discusses only titanium dioxide having surface areas of 50 and 100 square meters per gram. The article discusses the extrusion process for manufacturing titanium dioxide having the requisite high surface area and the effect of the raw materials, additives and process parameters employed in the extrusion process on catalytically important characteristics of the resulting titanium dioxide. As disclosed, the extrusion process involves the steps of (1) mixing and kneading the raw materials, (2) extruding, (3) drying, and (4) calcining, each of which influences the quality of the resulting support. Correlations between the concentration of water, plasticizers and binders and the type of titanium dioxide raw material employed in the mixing and kneading step and the crushing strength, attrition resistance, pore diameter and pore volume of the resulting catalyst support, and correlations between the calcination temperature and the surface area, pore volume, mean pore diameter and pore size distribution and the degree of transformation from the anatase crystalline phase to the rutile crystalline phase in the resulting catalyst support, are discussed in the article. More particularly, the use of catalysts containing palladium, platinum or rhodium components supported on titanium dioxide for selective hydrogenation is disclosed. On pages 240-241, the use of such catalysts to hydrogenate a para-substituted benzaldehyde to the corresponding para-substituted benzyl alcohol or para-substituted toluene is disclosed. The table on page 241 indicates that the para-substituent can be a carboxylic acid group, a methyl group or a halogen. The article discloses that the results of the hydrogenation of para-substituted benzaldehyde were substantially different depending upon whether the catalyst contained palladium, platinum or rhodium on the titanium dioxide support. The article indicates that the titanium dioxide must have a high surface area in order to be a suitable catalyst support and discusses only titanium dioxide having surface area of 50 and 100 square meters per gram. In addition, the article discloses that depending on the reaction temperature employed, the reduction of a para-substituted benzaldehyde affords either of several products with high selectivity and in high yield. Except for the catalysis, the reaction temperature and the hydrogen pressure employed, the article does not disclose the conditions under which the hydrogenation was performed.
Commonly assigned U.S. Pat. No. 4,743,577, to Schroeder et al, discloses that the use of catalysts containing palladium finely dispersed on carbon in the aforesaid purification-by-hydrogenation of terephthalic acid derived from the oxidation para-xylene results in contamination of the resulting purified terephthalic acid with fines produced by abrasion of the carbon granulates due to their relatively low crush strength and abrasion resistance. This patent discloses that reduced fines contamination results from the use instead of a catalyst containing a thin layer of palladium, nickel, rhodium, platinum, copper, ruthergum and cobalt on a porous sintered support of metallic titanium, zirconium, tungsten, chromium, nickel, and alloys incorporating one or more of these metals. The surface area of palladium-plated supports of titanium, Inconel and nickel are disclosed as 0.22, 0.55 and 1.21 square meters per gram, respectively, which are very significantly smaller than smaller surface area of a palladium on active carbon catalyst.
Commonly assigned U.S. Pat. No. 5,292,934, to Sikkenga et al., discloses the preparation of an aromatic carboxylic acid by the liquid phase catalyzed oxidation of an alkyl-substituted aromatic compound such as 2,6-dimethynaphthalene, ortho-xylene, meta-xylene, or para-xylene. The U.S. Pat. No. 5,292,934 further discloses that the resulting aromatic dicarboxylic acids can be purified by hydrogenation thereof in the presence of a catalyst comprising one or more Group VIII metals deposited on a support such as alumina, titania or carbon The application contains no other mention of titania.
Timruer et al., U.S. Pat. No. 4,831,008, describes using a catalyst containing a rhodium-containing component supported on titanium dioxide for the hydrogenation of benzene, toluene, ortho-xylene, terephthalic acid, disodium terephthalate, and diethyl terephthalate, in which the aromatic ring is hydrogenated.
Commonly assigned U.S. Pat. No. 4,892,972, to Schroeder et al., discloses that aqueous solutions of crude terephthalic acid can be purified by hydrogenation in the presence of plural noble metal catalysts in separate layers. Initially, the solution to be purified is passed through a layer of ruthenium-on-carbon catalyst, rhodium-on-carbon catalyst, or platinum-on-carbon catalyst, and thereafter through a layer of palladium-on-carbon catalyst, both under reducing conditions, i.e., while in the presence of hydrogen. Hydrogenation in the presence of plural noble metal catalysts in separate layers according to U.S. Pat. No. 4,892,972 involves conversion of 4-carboxybenzaldehyde to para-hydroxy-methyl-benzoic acid and/or to para -toluic acid and substantial conversion of 4-carboxybenzaldehyde to benzoic acid concurrently with a Fischer-Tropsch type of reaction in the same reaction vessel which converts generated carbon monoxide to a hydrocarbon moiety such as methane, ethane, or the like. Decarbonylation and carbon monoxide conversions to a hydrocarbon moiety are believed to occur substantially simultaneously in the first layer of the layered fixed catalyst bed of this invention. Such carbon monoxide conversions also require hydrogen. Thus the mount of molecular hydrogen supplied to the liquid-filled particulate bed is at least equal to (i) that stoichiometrically required to effect hydrogenation of a portion of the 4-carboxybenzaldehyde content of the aqueous solution to form para-hydroxymethyl-benzoic acid and/or para-toluic acid and (ii) that stoichiometrically required to form hydrocarbon moieties from a major portion of the carbon monoxide generated by decarbonylation to benzoic acid of another portion of the 4-carboxybenzaldehyde. The parent contains no mention of employing a noble metal supported on a carrier comprising titanium dioxide.
Recently, in commonly assigned U.S. Pat. No. 5,362,908, to Schroeder et at., a method employing a titanium dioxide-supported purification catalyst is disclosed for purification-by-hydrogenation of a crude terephthalic acid, crude isophthalic acid or a crude naphthalene dicarboxylic acid produced by the liquid-phase oxidation with an oxygen-containing gas in a solvent at an elevated temperature and pressure and in the presence of an oxidation catalyst comprising a heavy metal component. The purification-by-hydrogenation process according to U.S. Pat. No. 5,362,908 comprises passing an at least partially aqueous solution of crude aromatic dicarboxylic acid at a pressure sufficient to maintain the solution substantially in the liquid phase through a particulate catalyst bed in the presence of hydrogen. Particulate catalyst for this purification-by-hydrogenation process is a noble metal of Group VIII of the Periodic Table of Elements on a titanium dioxide support which does not disintegrate in less than one month under conditions employed in the hydrogenation. Preferably, at least one weight percent of the titanium dioxide support is in the rutile crystalline phase, and at least about 90 weight percent of the titanium support is, more preferably, in the rutile crystalline phase. The patent contains no mention of employing a noble metal supported on a carrier comprising titanium dioxide under conditions suitable for decarbonylation of organic impurities.
However, even after hydrogenation, the terephthalic acid product contains color bodies. It is highly desirable to reduce the concentration of such color bodies that remain in purified terephthalic acid. The color level of purified terephthalic acid product is generally measured directly either by measuring the optical density of solutions of purified terephthalic acid or the b*-value of the solid purified terephthalic acid itself. Optical density of purified terephthalic acid is measured as the absorbency of light at 340 nanometers in its basic solution, i.e., in a solvent such as sodium hydroxide or ammonium hydroxide.
Furthermore, even after hydrogenation, the terephthalic acid product often contains impurities which fluoresce at wavelengths of 3 nanometers upon excitation at wavelengths of 260-320 nanometers. Further reduction of such fluorescence of the purified terephthalic acid product is highly desirable. Since the concentration of such impurities in purified terephthalic acid can vary significantly, specifications are often established for the amount of such fluorescence which can be permitted for the purified terephthalic acid product. The problem of the control of such fluorescence by purified terephthalic acid is complicated because some of the fluorescent impurities are soluble and can be removed by conventional procedures for purifying terephthalic acid while other such fluorescent impurities are insoluble and cannot be removed by such conventional procedures. Upon chemical reduction during purification of crude terephthalic acid, some impurities which do not themselves fluoresce at wavelengths of 39 nanometers upon excitation at wavelengths of 260-320 nanometers are converted to their reduced forms which fluoresce at 39 nanometers upon excitation by wavelengths of 260-320 nanometers.
Regardless of the dicarboxylic aromatic acid desired, there is a need for improved catalytic processes for economical purification of relatively impure dicarboxylic aromatic acid produced by liquid-phase oxidation of a suitable benzene or naphthalene. Processes which can demonstrate treatment of higher levels of impurities without loss in quality of purified acid product should be very useful. Improvements which extend useful catalyst life could provide more economical purification processes. A catalytic process which extends useful catalyst life while treating crude acid having higher levels of impurities and provides improvements in quality of the purified acid product would be particularly advantageous.
It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods, for production of purified aromatic acids from liquid phase oxidation which can be used for manufacture of polyester fibers, polyester films, and resins in bottles and like containers.
More particularly, it is an object of the present invention to provide an improved method for production of purified aromatic acid sufficiently free of undesired impurities so that the acid can be used to make polyester resins which have good color and other properties needed in manufacture of commercial articles.
It is therefore a general object of the present invention to provide an improved method which overcomes the aforesaid problems of prior art methods, for purifying a crude phthalic acid or crude naphthalene dicarboxylic acid produced by the liquid-phase oxidation of ortho-xylene, meta-xylene, or para-xylene or a dialkylnaphthalene, respectively, with an oxygen-containing gas in a solvent and in the presence of an oxidation catalyst.
More particularly, it is an object of the present invention to provide an improved aforesaid purification method that employs a catalyst which does not produce particulates during the purification operation and yet has a high catalytic activity and lifetime.
It is another object of the present invention to provide an improved-aforesaid purification method that employs a catalyst that, even after a substantial period of aging, reduces the mounts of 4-carboxybenzaldehyde to substantially lower levels.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.